Catalytic and kinetic study of the liquid-phase oxidation of lactose over Au/Al2O3 nanostructured catalysts in a monolithic stirrer reactor

Abstract

The liquid-phase oxidation of lactose (LA) to lactobionic acid (LB) was studied over Au(2 %)/Al2O3 nanostructured catalyst. Catalytic tests were carried out at 65 °C, varying O2 partial pressures in the range of 0.21−1 bar and LA initial concentrations between 0.007 and 0.089 M. In all of the cases, the selectivity to LB was always 100 %. It was observed that the trends for the LA conversion (XLA) with time depends on the ratio of catalyst weight to initial LA moles W/nLA0 and the initial LA concentration CLA0. Experimental data were interpreted by kinetic modelling applying pseudo-homogeneous and Langmuir-Hinshelwood-Hougen-Watson (LHHW) models. It was found that irreversible surface oxidation of LA rules the reaction rate when W/nLA0= 30 g mol−1. At the early stages, the reaction rates did not depend on the LA concentration, and thus the reaction order with respect to LA was zero. In this case, an LHHW model considering total coverage of the active sites gives a very good fitting of the experimental data. As LA was consumed, the progress of conversion with time is different for each case and the reaction order changes to positive. Now, a better fitting is obtained with an LHHW model in which partial coverage of the active sites is considered. When liquid phase was diluted in LA and W/nLA0= 95 g mol−1, the LA chemisorption becomes the controlling step. Therefore, the results obtained by kinetic modelling helped to explain the dependence of LA oxidation rate with CLA0 and W/nLA0.